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Ganymede and Titan mass more than Mercury. Callisto, Io, our Moon, Europa, Triton, and Eris all mass more than Pluto. Are they planets? What is the value for this lower mass limit and what is it based on? Pluto masses 1.3 x 1022 kg, is that the limit, and if so, why that value?

My understanding of comets and asteroids is that the compositions are highly variable and even the difference between a comet and an asteroid is blurry - how do you define such that Ceres is not a planet, or should it be?

I'm not picking on you, nor am I favoring or arguing against any of these definitions. But I think a lot of people act like it is easy to come up with some definition, and that the IAU somehow missed some easy, obvious definition. I'm saying it is a lot harder.

I'm also saying that if your entire goal is to make Pluto a planet (a goal I completely fail to understand), then you are going to have to open it open to a lot of objects that were not traditionally planets.

My goal is to have a sensible taxonomic classification. The IAU's isn't, as a) it's explicitly limited to the Solar System and b) identical objects would change categories based on location. Pick a mass or a property. Hydrostatic equilibrium, composition, combination of the two all make sense.

Any classification should be based solely on the body's intrinsic properties.

Last edited by swampyankee; 2017-May-05 at 10:51 PM.

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My goal is to have a sensible taxonomic classification. The IAU's isn't, as a) it's explicitly limited to the Solar System and b) identical objects would change categories based on location. Pick a mass or a property. Hydrostatic equilibrium, composition, combination of the two all make sense.

Any classification should be based solely on the body's intrinsic properties.

But isn't that "b) identical objects would change categories based on location"?

Originally Posted by swampyankee

My goal is to have a sensible taxonomic classification. The IAU's isn't, as a) it's explicitly limited to the Solar System and b) identical objects would change categories based on location. Pick a mass or a property. Hydrostatic equilibrium, composition, combination of the two all make sense.

Titan, for example, is a planet if it orbits the sun, but a moon if it orbits Saturn?

Again, I'm fine with the classification changing depends on what it orbits (and the nature of that orbit), but I didn't think you were.

But isn't that "b) identical objects would change categories based on location"?
Titan, for example, is a planet if it orbits the sun, but a moon if it orbits Saturn?

Again, I'm fine with the classification changing depends on what it orbits (and the nature of that orbit), but I didn't think you were.

I can't communicate clearly on my tablet

If something meets some definition of "planet," based on its intrinsic properties when it's orbiting a star (not just the Sun, but any star), it would remain a planet if it orbits a planet or if it's wandering in intergalactic space orbiting nothing.

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If something meets some definition of "planet," based on its intrinsic properties when it's orbiting a star (not just the Sun, but any star), it would remain a planet if it orbits a planet or if it's wandering in intergalactic space orbiting nothing.

It certainly should do. The fact that there are definitions of "planet" that rule out candidates based on what they orbit does, in my opinion, make "planet" a useless term in any scientifically appropriate taxonomy of objects in space.

First mass, second composition, third surface characteristics, orbit irrelevant, would be my ordering.

Would either Titan or Ganymede "clear their orbits" had they been thrust into interplanetary space? I'm asking because I genuinely don't know.

It would depend on where you put them in orbit around the Sun. Right now, were any of the terrestrial planets moved sufficiently far from the Sun, they would fail the "clear its orbit" requirement, and change category. If not bound to the Sun, they would fail the "bound directly to the Sun" requirement.

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It would depend on where you put them in orbit around the Sun. Right now, were any of the terrestrial planets moved sufficiently far from the Sun, they would fail the "clear its orbit" requirement, and change category. If not bound to the Sun, they would fail the "bound directly to the Sun" requirement.

I was thinking more along the lines of them being part of the inner solar system. Not that our system is any standard bearer for how they are set up across the cosmos.

I was thinking more along the lines of them being part of the inner solar system. Not that our system is any standard bearer for how they are set up across the cosmos.

Somewhere or another on Cosmoquest, there's a link to a site which calculates a value based on a formula developed by Margot which will tell whether or not an object will clear its orbit. I suspect that either Ganymede or Titan, in direct orbit about the Sun, would satisfy the "clearing orbit" criterion out to a few AU.

Last edited by swampyankee; 2017-Jul-01 at 05:10 PM.

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Here's the thing, we all or most of us grew up 'knowing' Pluto is a planet. We humans like things orderly and Pluto should be a planet simply because it was discovered and counted as one for years. To say Pluto cannot be a planet because of it's size is like saying Israel cannot be a nuclear power because of it's size.

We mostly humans love 9nth planets that begin with the letter P. Sorry, but is true. And there is supposedly new evidence of a '9th planet' massing 10x earth.

To say Pluto cannot be a planet because of it's size is like saying Israel cannot be a nuclear power because of it's size.

That analogy makes no sense to me.

The nature of science is that we change as we get new information. Classifying animals by species constantly changes as new information comes along - look up how many times Northern Orioles and Baltimore Orioles went back and forth (LINK). Should we decide the validity of science by the criteria "well, that's not what it was when I grew up"?

Sure, let's debate about what definitions of planets are valid, useful, and are supported by the data. But setting it because someone memorized "My Very Elegant Mother Just Sat Upon Nine Pins" is silly.

Isn't this sort of like arguing whether a certain bit of rock is the world's biggest grain of sand or the world's smallest gravel granule?
Anyway, I would vote "planet". Just my three cents (inflation don't you know).

Isn't this sort of like arguing whether a certain bit of rock is the world's biggest grain of sand or the world's smallest gravel granule?
Anyway, I would vote "planet". Just my three cents (inflation don't you know).

There are explicit definitions which define gravel vs sand vs silt in civil engineering and geology. Interestingly, the ASTM definition doesn't change if the gravel is in California or Florida.

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